Hey there. I've been waiting quite long for such thing to happen, but then there is no such thing as secure software. As I've recently stated on my blog in other context: The only 100% security you have is physically disconnect. So if you can't deal with at most 80% security, you have to unplug your network cable. That's a harsh reality for some and I had the feeling that particularily some of the mac users made the impression that they are not aware that their system is vulnerable. But both you and me already know that we always have to be cautious.

Apple has been making some mistakes in the past and this is obviously one of the more scary ones. It's easy to fix though... just get ARDAgent inside a .zip and out of your way for now. Still, as you already stated, there are probably some time bombs ticking in the system. But then even in systems that are focused entirely on security, there are security vulnabilities... a programmer is only human and a software system is that complex that you can't always pinpoint every possible attack vector.

As for apple, there are some Ideas around that could help with that. Stack Smashing Protection, AddressSpace Layout randomization (i. e. completely, not partially as in leopard), complete sandboxing of processes, etc ... make it quite hard for attackers to exploit the hole in the grid they found. I just hope Os X is going to have them soon (latest in 10.6). But even then, you always have to be sure what you are installing.

Still, you always have to make sure that you know what you are doing if installing something, which isn't always a good idea.